As described in, for example, Zhe Li et al.,“SSBI Mitigation and the Kramers-Kronig Scheme in Single-Sideband Direct-Detection Transmission With Receiver-Based Electronic Dispersion Compensation”, JLT, vol.35, no. 10, May 15, 2017, there has been known a transmission system in which an IQ-modulated single polarization optical signal is transmitted together with local light and received by a single photodetector. Additionally, there has been known a method that reproduces an I-axis component and a Q-axis component from an optical signal by the heterodyne detection using a costas loop circuit as disclosed in, for example, Rafael Puerta, “Optically Generated Single Side-Band Radio-over-Fiber Transmission of 60 Gbit/s over 50 m at W-Band”, OFC Conference, 2017, M3E.4.
In such transmission systems, the adoption of the following transmission process allows the transmission rate to be twice the transmission rate in a case where a single polarized wave is transmitted. First, a polarization multiplexed optical signal in which two polarized components are multiplexed is transmitted. Then, the transmitted polarization multiplexed optical signal is demultiplexed into individual polarized components by a polarization beam splitter at the receive side. Thereafter, the demultiplexed polarized components are received by two photodetectors.